A number of methods for automatic focusing are known in the existing art. U.S. Pat. No. 5,483,079, for example, describes a method for focusing onto the surface of a light-transmitting specimen that is based on the pupil splitting principle. In this, the symmetry of the illumination at the focus point is analyzed using a special receiver. The energy center of the illuminated surface on the receiver is defined as the focus point. A method of this kind is very fast, but also has the disadvantage that it reacts sensitively both to inhomogeneities in the reflectance of the sample and to edges, such as those usually present e.g. on patterned wafers. Asymmetries in the illumination of the receiver are caused by the inhomogeneous reflectance or by scattered light produced at such pattern edges. These asymmetries in the light distribution or intensity distribution are interpreted by the focus measurement system as deviations from the ideal image plane, since the energy center point has a different location than in the case of homogeneous illumination. An attempt is made to compensate for these supposed deviations by displacing the image plane. This results in defocusing.
A further disadvantage of such focusing methods based on pupil splitting results from the manner in which they are usually implemented, namely as an optical system in addition to the main beam path: the light necessary for focusing is switched into the main beam path via a mirror, and also switched out again via another mirror. In other words, further components must also be made available in addition to the actual optical system in which the components arranged along the main beam path serve, for example, to analyze layer thicknesses. The light source used for the system that provides focusing is moreover usually a laser that operates in a narrow wavelength region in the infrared. This causes substantial decreases in reflectance with layer combinations of certain materials, so that focusing is no longer possible at all.
Other, so-called “intelligent” focusing methods use the image content itself as a criterion for focusing. In such methods, the image of the sample is generally imaged onto a CCD receiver and examined in terms of various sharpness criteria. This method, too, has a number of disadvantages. For one, the image must exhibit a structure so that the sharpness can be analyzed in the first place. Such is not the case for all samples, however. On the other hand, the capture region is very small and, as a result, the optimum focus point is difficult to find. The reason for this may be found in the manner in which the method is carried out: when a measurement location on the sample is traveled to with a measuring instrument, the deviation from the focus point is usually so great that all the structures in the image field are extremely unsharp, and no criteria for sharpness can be derived from them. The direction in which the image plane must be displaced to make the image sharper is, moreover, initially unknown. These problems are solved by acquiring a series of images in a sequence of image planes while traversing a large focus region, and then assuming that the image plane of the image having the greatest sharpness is the focal plane, i.e. the image plane containing the focus point. This does not, however, necessarily find the focus point, whose image plane need not inevitably be contained in the previous acquired sequence. A method of this kind is thus unsuitable when it becomes necessary to perform measurements on a focused specimen, for example using spectrophotometers in layer thickness determination.
An automatic focusing system that likewise utilizes image processing but is said also to be suitable for unstructured surfaces is presented in U.S. Pat. No. 5,604,344. In the method described therein, a special structure is imaged onto the surface of the measured specimen and then analyzed in terms of its sharpness. With this method, problems once again crop up when interactions occur between the imaged structures on the one hand and structures in the measured specimen on the other hand.
The problem of determining the direction in which focusing is to occur is solved in a variant embodiment of U.S. Pat. No. 5,604,344 by the fact that several structures located in different image planes are used. The disadvantage of the small capture region (and thus relatively long focusing times) nevertheless remains, however, since a large region must be repeatedly searched.
U.S. Pat. No. 5,747,813 describes another method that uses an asymmetrical measurement beam path, i.e. a beam path in which the incidence angle of the light onto the sample surface differs from 0°, for examination of the sample. As a consequence, the location of the image field on a position-sensitive detector (for example, a video camera) shifts along a straight line when the distance between the imaging optical system and the sample is adjusted. With a homogeneously illuminated image field, focusing is said to be achieved when the image field is located centeredly on the receiver; the recorded intensity is used for fine adjustment, and the focus point corresponds to a minimum of the recorded intensity. Inhomogeneities in the reflectance of the sample such as those brought about e.g. by structures can, however, easily cause distortion of this analysis so that focusing is unsuccessful.